In order to reduce the amount of space taken up by the main rotor of a helicopter as parked on a parking area, on the deck of a ship if the helicopter is an on-board helicopter, or else stowed in a hangar, it is known to equip helicopters with rotors in which each blade is foldable.
Generally, the blade is folded about a folding axis situated at the radially outer end (relative to the axis of the rotor) of a coupling member, called coupling sleeve serving to couple the root of the blade to the hub, and itself being connected to the hub by retaining and hinging means whereby the blade can make pitch, flap, and drag angular movements.
Such movements, and in particular pitch angular movements, must be prevented while the blades are being folded.
If a blade is not prevented from pitching while it is being folded, the flapping mass constituted by the blade and its associated sleeve tilts about its pitch-changing axis. Such tilting can be dangerous for the operators working around the helicopter, and for the corresponding retaining and hinging means for the blade, which means might be damaged.
In addition, gusts of wind or movements of the ship on board which the helicopter is situated can cause the flapping masses to be displaced, which might damage the constituent parts of the rotor, or be dangerous for the operators.
To mitigate those drawbacks, methods of folding blades have already been proposed in which the blades are prevented from pitching firstly by disuniting the pitch rods from the pitch levers. Then each blade sleeve is angularly positioned to the required folding inclination, and the sleeve is secured to a member of the hub that is prevented from pitching. The pitch rods are then retained with blade pin clips placed firstly across the necks of devises or in bores in the pitch rods, and secondly in bores with ball joints on the pitch levers.
That folding method is quite satisfactory as regards preventing the blades from pitching, but, when the blades are deployed, it makes it necessary to put back into place the fixing means for fixing each pitch lever to the corresponding pitch rod, so as to ensure that the pitch of each blade is set.